Because of the energy crisis beginning in the mid-1970's and due to economic factors within the electric utility industry, there is a need for storing bulk quantities of electrical power which might be produced intermittently or randomly by devices such as wind-driven generators, solar cells or the like. A number of methods have been considered including the storage of compressed air in large reservoirs, flywheels, capacitive storage, inductive storage and a number of electric chemical schemes. Electrochemical storage batteries are generally expensive, heavy and subject to deterioration when subjected to repeated charge and discharge actions.
Up until now, only pumped water storage wherein water from a water storage pond at one level is directed to a water storage pond at a lower level through a hydro-electric plant having a water pumping capability has proven to be a viable method. Unfortunately, such facilitites are limited to areas where the terrain is suitable for providing water sources at different elevations.
Electrically rechargeable REDOX flow cell systems are well known and have a very high overall energy efficiency as compared to many other systems. Furthermore, REDOX type cells can be discharged more completely than secondary battery systems. Additionally, REDOX cells are inexpensive as compared to secondary batteries and do not deteriorate as significantly when repeatedly discharged or recharged.
Accordingly, it is an object of the present invention to provide a REDOX cell which acts as a bulk energy storage system of very high overall efficiency.
Another object of the present system is to increase the rates of charging and discharging of REDOX flow cells that use the soluble chromous/chromic REDOX couple.
A further object of the present invention is to provide electrode structures of a REDOX cell which are catalytic for the oxidation of chromous ions to chromic ions, catalytic for the reduction of chromic ions to chromous ions and highly irreversible for the hydrogen evolution reaction.
Still another object of the present invention is to provide a REDOX cell wherein the active materials are not present in the cell at all times.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
In accordance with the present invention, zirconium carbide is used as a catalyst for the oxidation of chromous ions to chromic ions in a REDOX cell. It is also used as a catalyst for the reduction of chromic ions to chromous ions in a REDOX cell. The zirconium carbide is coated on an inert, electrically conductive electrode.
The general purpose of the present invention is to increase the rates of charging and discharging of REDOX flow cells that use the soluble chromous/chromic REDOX couple. The reversible half-cell potential for this REDOX couple is about -0.4 volts with respect to the hydrogen evolution potential. Upon recharge of a REDOX flow cell where chromium ions are being transformed from the plus three valence state to the plus two valence state, the thermodynamically favored electrochemical reaction would be the evolution of hydrogen. However, the kinetics of the hydrogen evolution reaction are slow on many surfaces. The desired electrode surface for the chromous/chromic reaction is one that would have a high hydrogen overvoltage (slow kinetics for hydrogen evolution) and yet at the same time be highly reversible for the electrochemical oxidation and reduction of chromium ions.
Previously, carbon and graphite materials were used as electrode structures. These materials permitted the desired results to be obtained; that is, chromic ions could be recharged to chromous ions with only a minimum fraction of the charging current being consumed by the undesired reaction of hydrogen evolution. However, the rate at which the reduction of chromic ion (Cr.sup.+3 +e.sup.- .fwdarw.Cr.sup.+2) took place while still maintaining a high charging efficiency (&gt;95%) was very low. A charging rate of about 3 ma/cm.sup.2 was typical for cells that used graphite cloth as the chromium electrode when an overvoltage of 100 milli-volts was applied to the cell. Further, the rate at which current could be withdrawn from a fully charged chromous chloride solution indicated that carbon and graphite surfaces were not catalytic for the oxidation to chromic chloride solution.
The material zirconium carbide was found to be catalytic for the oxidation of chromous ions to chromic ions, catalytic for the reduction of chromic ions to chromous ions as well as being highly irreversible for the hydrogen evolution action.